When a laser beam, called a pump laser beam, is transmitted through a non-linear crystal, due to the interaction between the laser light and the crystal, there exists some probability that a pump laser photon, with angular frequency ω0, is annihilated giving rise to a photon pair with different angular frequencies. The angular frequency ω is connected to the (ordinary) frequency ν by ω=2π ν. In the following, the term frequency will also be used for the angular frequency ω. Moreover, the angular frequency ω is connected to the corresponding wavelength λ by λω=2πc, where c is the speed of light.
In accordance with the conservation of energy and momentum, the total energy and the total momentum of the two produced photons are equal to the energy and momentum, respectively, of the annihilated photon. The above described process of producing photon pairs using a non-linear medium is called spontaneous parametric down-conversion, SPDC. The relation between the frequencies of the photons as well as the wave vectors of the photons may be described as:ω0=ω1+ω2 conservation of frequency, i.e. energyk0=k1+k2 conservation of wave vector, i.e. momentumHere ω0 denotes the frequency of the pump photon, ω2 denotes the frequency of the so called signal photon and ω1 denotes the frequency of the so called idler photon. Due to energy conservation, the frequencies ω2 and ω1 are lower than the frequency ω0. Similarly, k0 denotes the wave vector of the source photon, k2 and k1 denote the wave vector of the signal photon and idler photon, respectively. It should be noted that the two correlated photons may be spatially separated.
Photon pairs produced in non-linear media may be used for imaging. So-called quantum ghost imaging, cf. EP 2 058 677 A1, utilizes photon pairs produced by the interaction of a laser with a non-linear crystal, e.g. SPDC. One of the down-converted photon beams, also called photon fields or just fields, illuminates the object and is detected using a photon counter with no spatial resolution. This detection is used to herald a spatially resolving detector placed in the other one of the down-converted beams. Although only one of the down-converted fields illuminates the spatially resolving heralded detector, e.g., a triggered camera, the ghost image is seen in amplitude correlations between the twin beams. Hence the detection of both beams is always necessary. However, detecting the photons that illuminate the object may be problematic. For some applications, this requirement proves to be a major drawback of this imaging technique because there are not always detectors available at the wavelength necessary for the illumination of the object. The problem with using triggered cameras is that the detection of the signal photons on the camera must be triggered by the detection of its brother idler photon. The idler is usually detected with a photon counter and that signal is used to herald detection on the camera. Hence, signal and idler photons must both be detected within a small time window. Because of the relatively slow electronics, this implies that the signal be put in a long image preserving delay line, to give time for the idler to be detected and for this information to arrive at the camera at the same time as the signal photon arrives at the camera. This is a major drawback. This also implies that one needs excellent detectors for both of the photon wavelengths that are produced. However, MIR photon counters usually have high dark counts, such that these are not easy to use to trigger a camera.
Optical parametric amplification, OPA, cf. P. M. Vaughan and R. Trebino. “Optical-parametric-amplification imaging of complex objects.” Optics Express, Vol. 19, Issue 9, pp 8920-8929 (2011), is another imaging technique. An object is illuminated with light from a pump laser and the light that illuminates the object is then sent through a non-linear crystal together with intense light from the pump laser. Photons generated by this process contain the image and are often at different wavelengths than the illumination pump laser. However this technique is rather complicated. In particular, the source has to be at the illumination wavelength the target should be illuminated with. This may be difficult to provide, especially when it comes to sources at non-visible wavelengths, of which only few are available. Moreover, this technique relies on a stimulated nonlinear process and therefore requires intense light beams. However, there may be many application areas such as imaging or studying fragile samples, e.g. biological samples, paintings etc. for which using intense light beams may not be needed or rather not wanted, since the high intensity would damage the sample.
One example may be organic compounds which exhibit specific absorption properties in the mid infrared, MIR. In medical imaging this could be relevant for cancer diagnostics, for example. MIR imaging may also be used in cultural heritage and paintings conservation. MIR imaging has additional applications in industrial imaging and security fields.
Imaging in the MIR regime may require sophisticated cameras. Such MIR cameras may be very demanding—a commonly used technical approach is based on cryogenic InSb semiconductor technology, cf. Rogalski, A. Infrared Physics & Technology 43 (2002) 187-210. MIR cameras are expensive and the InSb semiconductor technology inside needs to be cooled down to very low temperatures. However, these cameras still feature a lot of noise and often exhibit poor signal-to-noise ratios.
Other approaches to imaging may be based on the inverse process to down-conversion, namely nonlinear up-conversion. This technique requires relatively strong pump laser power, and a light source at the illumination wavelength. In up-conversion imaging, images are nonlinearly converted to a wavelength regime in which efficient and low-noise detectors may be available. Due to the requirement of strong pump lasers, in order to be efficient, using this technique for many applications is hampered if not impossible.